Heat in California, June 18 to 25, 2017

Name: Heat in California, June 18 to 25, 2017
Start: 2017-06-18T11:00
End: 2017-06-25T20:00
Location: California

June 18 to 25, 2017 · NWS HNX · 10 events in 9 counties and zones

The toll

Deaths: 0
Injuries: 0
Property damage: None recorded
Crop damage: None recorded
Events: 10

Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.

What happened

The episode as NWS HNX wrote it up.

A large area of high pressure began to build north and west into central California on June 13 and strengthened over the area through June 17 with temperatures rising to well above seasonal normals. This led to a heat wave which resulted in high temperatures often exceeding 105 degrees across much of the San Joaquin Valley between June 18 and June 25 and exceeding 110 degrees across the Kern County Deserts. Excessive Heat Warnings were posted for these areas as well as for the southern Sierra foothills and the Kern County Mountains where temperatures were much above normal for much of this period.

What forecasters said that day

The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Hanford, CA wrote its last area forecast discussion before the first event at Jun 18, 2:59 AM PST, 8 hours 1 minute earlier.

Area forecast discussion, Jun 18, 2:59 AM PST, 8 hours 1 minute before

Area Forecast Discussion
National Weather Service San Joaquin Valley - Hanford CA
359 AM PDT Sun Jun 18 2017

.SYNOPSIS...A strong ridge of high pressure will bring hot
weather to the central California interior through at least
Thursday with near record temperatures. Isolated afternoon
thunderstorms are possible along the Sierra crest each day
through Tuesday.

&&

.DISCUSSION...Exactly one week ago, an unseasonably cool air mass
blanketed the Golden State. For residents of the central
California interior, last Sunday was one of the coolest June days
on record. The pattern has done a complete turnaround since then.
Beginning today and for much of this week, residents of the San
Joaquin Valley, lower foothills and the Kern county desert will be
roasting in triple digit heat. Temperature records in the valley
will be challenged each and every day through Thursday. Monday
through Wednesday will be the hottest days as thermometer readings
top 110 degrees in several lower elevation locations. Nightly
minimum temperatures will be unseasonably warm, too, and also
possibly record-breaking, with lows generally in the 70s to lower
80s. It's hard to believe that one week ago, those were the high
temperatures in the San Joaquin Valley!

The primary blame for this heat wave is a very strong upper level
ridge of high pressure. It is currently centered over California.
After today, the center of this ridge will migrate to Arizona
where it will remain anchored through the end of next week. On a
three dimensional view, these ridges are like very tall mountains
of very warm, dry air, and they are very difficult to displace.
The models often struggle with this concept and usually try to
break these ridges down too quickly and in turn trend temperatures
toward climatological normals in the extended forecast periods.
For the past few days, successive model runs have slowed the
breakdown of this upper level ridge and delayed the downward
trend in temperatures. The latest run now shows no change in
500 MB heights or 850 MB temperatures across the central
California interior through Thursday. Hence, the reason we've
extended the Excessive Heat Warning through day 5. It would not
be a surprise if future model runs show little change on days
6 and 7, too. Whatever the case, temperatures Friday through next
weekend will not be as hot, but still well above normal.

Otherwise, the air mass residing over the central California
interior will stagnate this week. Inevitably this will lead to
worsening air quality in the San Joaquin Valley. Humidities
have also increased slightly, too, especially in the southeastern
San Joaquin Valley. While this could partly be due to irrigation,
it's more likely due to the fact that many rivers are running
full and will remain so through at least the middle of Summer.
Our near record warmth this week will significantly increase
snowmelt over the high Sierra and raise water levels on most
rivers. Minor flooding of the Merced river in Yosemite National
Park is likely the next two or three nights as a result. It
can't be emphasized enough how very dangerous all rivers in
California are. These rivers are fed by melting snow, so they
are running very cold, deep and swift. Too many drownings and
water rescues have already occured this Spring because people
foolishly chose to cool off in these rivers.

Additionally, the moisture available from melting snow over the
high Sierra could help generate isolated thunderstorms along the
crest the next few afternoons.  The air mass is pretty stable,
so it may take until very late in the day for towering cumulus
to break through the inversion cap. Nonetheless, hikers in the
Sierra high country should keep a watchful eye on the sky the
next few afternoons.

&&

.AVIATION...
Isolated thunderstorms are possible along the Sierra crest
between 20z today and 03z Monday. Otherwise, VFR conditions
will prevail across the Central California interior during the
next 24 hours.

&&

.AIR QUALITY ISSUES...
On Sunday June 18 2017... Unhealthy for sensitive groups in
Fresno... Kern... Kings... Madera and Tulare Counties and Sequoia
National Park and Forest.
Further information is available at Valleyair.org

&&

.CERTAINTY...

The level of certainty for days 1 and 2 is high.
The level of certainty for days 3 through 7 is high.

Certainty levels include low...medium...and high. Please visit
www.weather.gov/hnx/certainty.php for additional information
an/or to provide feedback.

&&

.HNX Watches/Warnings/Advisories...
Excessive Heat Warning from noon today to 11 PM PDT Thursday
CAZ089>095-098-099.

&&

public...Durfee
avn/fw...Bean
synopsis...Durfee

weather.gov/hanford